f-strings — Building Readable Output

By the end of this lesson you can:

• Write an f-string that drops variables into a sentence.
• Round a number to two decimal places inside the f-string with :.2f.
• Pad text and numbers to a fixed width so a column lines up.

Two ways to print the same line. Look closely at the spacing, and predict what the third line does.

item = "Hot dog"
price = 8.5

print("Item:", item, "costs $", price)
print("Item: " + item + " costs $ " + str(price))
print("Item: " + item + " costs $ " + price)

Item: Hot dog costs $8.5
Item: Hot dog costs $8.5
Traceback (most recent call last):
TypeError: can only concatenate str (not "float") to str

Like filling in a form. The sentence is printed already, with blanks; you write a name in one blank and a price in another. The f before the quote tells Python the blanks are live.

item = "Hot dog"
price = 8.5
qty = 3

print(f"Item: {item} costs $ {price}")
print(f"{qty} sticks of {item} = $ {price * qty}")

Output

Item: Hot dog costs $8.5
3 sticks of Hot dog = $25.5

Three things to notice:

• The f is essential. Without it you get the literal text {item}, braces and all. No error — just wrong output.
• Numbers need no conversion. Python calls str() for you inside the braces.
• Braces can hold a calculation. {price * qty} works, as does any expression.

Money needs two decimal places

$25.5 is not how money is written, and $7.700000000000001 is worse. Add :.2f inside the braces.

price = 8.5
total = 2.2 + 5.5

print(f"Bad:  $ {price}")
print(f"Good: $ {price:.2f}")
print(f"Bad:  $ {total}")
print(f"Good: $ {total:.2f}")

Output

Bad:  $8.5
Good: $8.50
Bad:  $7.7
Good: $7.70

Read :.2f as "format as a decimal number with 2 places". It rounds for display only — the variable itself is untouched.

Widths line a column up

A number after the colon reserves that much room. Text pads to the left, numbers to the right, which is exactly what a price column needs.

print(f"{'Item':<12}{'Qty':>4}{'Price':>9}")
print(f"{'Pizza':<12}{2:>4}{7.00:>9.2f}")
print(f"{'Hot chocolate':<12}{10:>4}{25.00:>9.2f}")

Output

Item         Qty    Price
Pizza     2     7.00
Hot chocolate     10    25.00

• {name:<12} — left-aligned in 12 characters.
• {qty:>4} — right-aligned in 4.
• {price:>9.2f} — right-aligned in 9, with 2 decimals. Width first, then the format.

Why it matters. Every receipt, report and table you print needs this. Ragged columns are hard to read; two extra characters in the braces fix them.

Three orders, printed as a table that lines up whatever the names are.

# order_summary.py - one aligned table, built with f-strings

orders = [
    ["Pizza", 2, 3.50],
    ["Hot chocolate", 3, 2.50],
    ["Pancake Telur", 1, 4.20],
]

header = f"{'Item':<20}{'Qty':>5}{'Unit':>9}{'Total':>10}"
print(header)
print("-" * len(header))

grand_total = 0
for item, qty, unit in orders:
    line_total = qty * unit
    grand_total = grand_total + line_total
    print(f"{item:<20}{qty:>5}{unit:>9.2f}{line_total:>10.2f}")

print("-" * len(header))
print(f"{'GRAND TOTAL':<20}{'':>5}{'':>9}{grand_total:>10.2f}")
print(f"\nThank you! Please come again.")

Output

Item                  Qty     Unit     Total
-------------------------------------------
Pizza              2     3.50      7.00
Hot chocolate               3     2.50      7.50
Pancake Telur        1     4.20      4.20
-------------------------------------------
GRAND TOTAL                            15.70

Thank you! Please come again.

Worth stealing from this:

• The header sets the widths. Every row uses the same numbers, so the columns cannot drift apart.
• "-" * len(header) draws a rule exactly as wide as the table, whatever you change the widths to.
• {'':>5} prints an empty column to keep the total under the right heading.

An f-string is a normal string with an f in front, where anything in braces is worked out and dropped in. After a colon you can control the display: .2f for money, a number for width, < and > for alignment. Formatting changes what is printed, never the variable.